Blind fasteners, or rivets, as well known in the art, are utilized in fastening components in which only one side of the workpiece is accessible. U.S. Pat. No. 3,148,578 to Gapp describes a fastener assembly comprising a fastener body in the form of a stem within the fastener body. One end of the stem has serrations for engagement by a pulling tool. The opposite end of the stem has an upsetting head. Adjacent the tail end of the sleeve is a shear ring, integrally formed with the stem.
In operation, the Gapp assembly is inserted through aligned holes in the superposed workpieces to be secured together. A tool is used to pull the stem axially away from the workpiece, while a reactionary force is applied against a head of the sleeve. During this first part of the pulling operation, the shear ring axially compresses the sleeve which causes radial expansion of the tail end of the sleeve and bulbing on the blind side of the superposed plates. The shear ring is constructed to shear from the stem at a predetermined tension on the stem, thereby limiting the amount of clamping compression on the superposed plates to a predetermined value. Continued pulling causes a lock ring groove in the stem to reach the sleeve head where a lock ring is deformed into the groove. Pulling increases the load, causing the stem extending beyond the accessible face of the workpiece to break away.
Shear rings integrally formed with the mandrel stem suffer from many manufacturing disadvantages. In one method of forming a shear ring integral with the stem, a piece of material having a diameter at least equal to the desired size of the shear ring is machined using cutting tools to form the shear ring. Such a process is not only time consuming, but also produces a large amount of scrap, or wasted material. In addition to the difficulties associated with the actual formation of the integral shear ring, there are also problems associated with tool wear. As the cutting tools become dull and worn, slight variations in the dimensions of the shear ring tend to appear. This is undesirable in that such variations make the fastener somewhat less reliable because the forces required to bulb the sleeve and shear the shear ring are less predictable. Thus, frequent tool sharpening or tool replacement is needed.
It is possible to form stems with integral shear rings by heading or coining but this is somewhat difficult because of the wide variation in diameters and because of an acute angle adjacent the ring.
Because the shear ring of a blind rivet is designed to shear at a predetermined force after forming the blind head, limitations are necessarily imposed on the axial width of the shear ring. In order to carve out a wedge shaped portion of the sleeve, prior integral shear rings have been manufactured with an included angle at the base of the shear ring, proximate the mandrel stem. This included angle portion of the shear ring presents a cutting surface on the leading edge of the shear ring. In operation, this type of shear ring first gouges the tail end of the sleeve so as to create a wedge therein. This wedge facilitates bulbing of the sleeve. After forming a blind head, the shear ring is sheared from the stem. Such an operation requires a large initial pulling force, and thins the walls of the tail end of the sleeve.
U.S. Pat. No. 3,390,601 - Summerlin discloses a blind fastener employing a separate collar which is positioned between an enlarged portion on the blind end of a stem and the blind end of the sleeve. The leading portion of the collar extends inwardly adjacent to a shoulder formed by the enlarged end of a stem and an adjacent reduced diameter portion of the stem. Forming the collar as a separate element, as opposed to having it formed integral with the portion of the stem, as in the above-mentioned Gapp patent, has the advantage of allowing the collar to be made of material different from the stem and to be fabricated separately from the stem. This, in turn, provides manufacturing advantages A major disadvantage of the Summerlin fastener, however, is that the necessary lock ring groove in the stem extends inwardly from a reduced diameter portion of the stem. Consequently, a break groove in the stem, which causes the stem to break after it is installed, has to have a diameter which is yet smaller than the lock groove. Consequently, this construction undesirably limits the load which may be applied to the stem. Further, the important shear strength of the fastener is reduced to the combined shear strength of the stem at its breakneck groove and the surrounding softer sleeve. Also, modifying the design to increase its shear strength would appear to undesirably increase the blind side protrusion of the stem. This may prevent installation of such a modified fastener in situations wherein space is limited on the blind side of the fastener.
In view of the foregoing, a need exists for an improved blind fastener of the type incorporating a shear ring in the fastener setting operation.